When SerieScanner encounters a folder without a local key or data file,
it now optionally falls back to a database lookup by folder name. This
prevents newly-added series from being silently skipped on rescan when
their metadata only lives in the DB.
Changes:
- SerieScanner accepts an optional db_lookup callable
- SeriesApp forwards db_lookup to SerieScanner
- AnimeSeriesService adds get_by_folder_sync() helper
- dependencies.py wires a sync DB lookup into get_series_app()
- Unit tests cover fallback hit, miss, and exception paths